Bioabsorbable interference screw for endosteal fixation of ligaments

ABSTRACT

A bioabsorbable interference screw which, upon insertion, the screw engages cortical bone at the back end of the bone tunnel and fills all but 5-10 mm. of the tunnel, thereby providing increased fixation strength while also promoting fast healing. The screw includes a head provided with a specially designed drive socket with radially extending slots at its outer end for receiving corresponding protrusions on the shaft of screwdriver. The drive socket optimizes the torque capacity of the screw.

This application is a continuation of application Ser. No. 11/097,179,filed Apr. 4, 2005, which is a continuation of application Ser. No.10/634,807, filed Aug. 6, 2003, now U.S. Pat. No. 6,875,216, which is adivision of application Ser. No. 09/711,964, filed Nov. 15, 2000, nowU.S. Pat. No. 6,629,977, which claims the benefit of U.S. ProvisionalApplication No. 60/165,722, filed Nov. 15, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to endosteal fixation of a substituteligament and, more specifically, to arthroscopic endosteal fixation of asubstitute anterior cruciate ligament using a tapered bioabsorbableinterference screw.

2. Description of the Related Art

When a ligament becomes detached from a bone, surgery usually isrequired to reconstruct the ligament. Often, a substitute ligament orgraft is secured into bone tunnels to facilitate incorporation andpermanent attachment.

Various methods of graft attachment are known, including the use ofinterference screws to secure the graft against the walls of a tunneldrilled through the tibia and a socket formed in the femur. A stronggraft attachment is obtained by using a metal interference screw towedge a graft bone block to the wall of a graft tunnel formed throughthe bone, as disclosed in U.S. Pat. No. 5,211,647 to Schmieding. If abioabsorbable interference screw is used, the graft is often wedgeddirectly against the bone by the screw, without a bone block.

Bioabsorbable interference screws are usually sized so that they areslightly larger that the diameter of the tunnel, so that they dilate thebone tunnel upon insertion. Dilation advantageously compacts the softcancellous bone between the ends of the tunnel, providing betterfixation. Conventional straight-sided bioabsorbable interference screwshave an interference fit of about 1 mm., i.e, about 1 mm. of bone isdilated as the screw is inserted into the bone tunnel. Although it wouldbe desirable to use larger diameter screws for increased fixationstrength, larger screws have larger tips and are more difficult to alignand insert correctly. Accordingly, a need exists for a bioabsorbableinterference screw which provides increased dilation and interferencefit without increased difficulty of insertion.

SUMMARY OF THE INVENTION

The present invention overcomes the disadvantages of the prior art andachieves the foregoing objectives by providing a bioabsorbableinterference screw which, upon insertion, fills all but 5-10 mm. of thelength of the tunnel, thereby providing increased fixation strengthwhile also promoting healing.

The bioabsorbable interference screw of the present invention includes ahead provided with a specially designed Delta drive socket for receivinga Delta drive screwdriver or a traditional hex-head screwdriver. Theunique drive socket of the interference screw of the present inventionoptimizes the torque capacity of the screw.

The bioabsorbable interference screw of the present invention ispreferably threaded along substantially the entire length of the screwto maximize fixation strength within the tunnel. Preferably, the distalend of the screw, the end closest to the joint, has a smooth, roundedtip profile so as to minimize abrasion with the graft.

The interference screw of the present invention may be optionallyprovided with a cannulation for insertion over a guide pin. In thisembodiment of the invention, a cannulated Delta drive or hex drivescrewdriver is used to insert the screw into the tunnel over the guidepin.

The bioabsorbable interference screw of the present invention ispreferably formed of highly crystalline poly-(L-lactic acid) (PLLA)compound.

In the preferred method of ACL reconstruction of the present invention,the graft, preferably a hamstring tendon autograft or allograft, issecured, preferably by interference screw fixation, in a femoral socketformed through the tibial tunnel, as described, for example, in U.S.Pat. No. 5,320,626, the disclosure of which is incorporated herein. Thehamstring graft is then drawn taut and secured in the tibial tunnel byinsertion of the tapered bioabsorbable interference screw of the presentinvention. If the interference screw is fully cannulated, a guide pinmay optionally be employed to guide the interference screw duringdelivery and installation.

Other features and advantages of the present invention will becomeapparent from the following description of the invention which refers tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional detail view, drawn along line A-A of FIG. 2,of the bioabsorbable interference screw of the present invention;

FIG. 2 is a rear elevational view of the bioabsorbable interferencescrew of the present invention;

FIG. 3 is a cross-sectional detail view, drawn along line C-C of FIG. 2,of the interference screw of the present invention;

FIG. 4 a cross-sectional detail view, drawn along line D-D of FIG. 2, ofthe interference screw of the present invention;

FIG. 5A is a side view of the interference screw driver and FIG. 5B is adetailed view of the tip of the driver; and

FIG. 6 shows the interference screw of the present invention beinginserted into the tibial tunnel against a ligament graft.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a cross-sectional view of the bioabsorbableinterference screw 10 of the present invention is shown. Screw 10 ispreferably formed of a bioabsorbable material, such as PLA or PLDLA(Poly(L/D-lactide)Acid). Screw 10 has a main body portion 15, a proximalend 20, and a distal end 25, and is preferably provided with a cannula30.

Screw 10 is provided in a preferred length of 35 mm., with threads 16extending substantially from proximal end 20 to distal end 25. The edges17 of the thread(s) 16 are flattened to prevent severing tissue duringscrew insertion.

Referring to FIG. 2, the proximal end 20 of screw 10 is provided with anelongated socket 35 configured to receive a Delta drive screwdriver 56,described below. To that end, socket 35 is provided, at its outer end,with radially-extending slots 40 in every other annular face of socket35. The slots 40 receive correspondingly-shaped protrusions 42 (shown inFIGS. 5A and 5B) on the proximal end of the shaft 58 of driver 56. TheDelta drive socket 35 permits increased torque capacity while minimizingthe problem of stripping the drive portion of the screw 10.Advantageously, the Delta drive socket can also be used with atraditional hex drive screwdriver.

The interference screw of the present invention is preferably providedin four sizes: (1) a screw which tapers from a 7.5 mm. diameter at itstip to 9 mm. at the socket; (2) a screw which tapers from a 8.5 mm.diameter at its tip to 10 mm. at the socket; (3) a screw which tapersfrom a 9.5 mm. diameter at its tip to 11 mm. at the socket; and (4) ascrew which tapers from a 9.5 mm. diameter at its tip to 12 mm. at thesocket.

As screw 10 threadingly advances through a bone tunnel, the screwdilates bone outwardly around the bone tunnel and creates aninterference fit therewith. The present interference screw promotesabout a 1.5 mm interference fit; i.e., the diameter of the proximal end20 of the screw 15 is 1.5 mm larger than the diameter of the bonetunnel. Typical bone screws, which are not tapered, provide a maximum of1.0 mm interference fit. The additional interference provides 28% morepull out strength.

Screw 10 is configured to be sufficiently long so as to fill all but thetop 5-10 mm of the tibial bone tunnel. This configuration secures alarge portion of the ligament graft against the bone tunnel while alsoproviding threading engagement of the threads 16 of screw 10 againstcortical bone at outer end of the bone tunnel. Because cortical bone issignificantly harder than the interior soft, cancellous core, corticalbone provides significantly more load bearing capability. As a result,the invention eliminates the need for multiple, shorter interferencescrews in a bone tunnel.

FIGS. 5A and 5B show the driver 56 for inserting interference screw 10.Driver 56 has an taped elongated hexagonally shaped shaft 58 at itsdistal end, best shown in the magnified view of FIG. 5B, which isprovided with protrusions 42 to mate with the Delta drive recess 35 ofscrew 10.

Referring to FIG. 6, the method of endosteal fixation of a ligamentgraft using the bioabsorbable interference screw of the presentinvention includes the steps of securing one end of a graft 60 in thefemoral socket 62, pulling the opposite end of the graft 60 (extendingthrough the tibial tunnel) taut, and fixating the graft 60 in the tibialtunnel 64 by mounting the bioabsorbable interference screw 10 on driver56 and, using the driver, driving screw 10 in the tibial tunnel againstgraft 60 to the level of the anterior cortex in the distal portion ofthe tibial tunnel, such that the interference screw fills all but thetop 5-10 mm. of the tunnel. Driver 66 is then removed, leaving screw 10in place with an interference fit of up to 1.5 mm.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein, but only by the appended claims.

1. A bioabsorbable interference screw for ACL reconstruction,comprising: an elongated threaded bioabsorbable body having a proximalend, a distal end, and a length of 35 mm for substantiallylongitudinally filling a tibial tunnel, the threads of the elongatedthreaded bioabsorbable body extending along substantially the entirelength of the elongated threaded body, the screw having a widthdimensioned to provide an interference fit in the tibial tunnel; a tipdisposed at the distal end of the elongated body, the tip having a taperto facilitate insertion of the screw into the tibial tunnel; and a drivesocket disposed within the screw and extending from the proximal end ofthe elongated threaded body, wherein the drive socket includes aplurality of radially extending slots configured to receivecorresponding radially extending protrusions on a shaft of ascrewdriver.
 2. The bioabsorbable interference screw of claim 1, whereinthe screw is fully cannulated for receiving a guide pin.
 3. Thebioabsorbable interference screw of claim 1, wherein the screw has adiameter of 9 mm. at the drive socket.
 4. The bioabsorbable interferencescrew of claim 1, wherein the screw has a diameter of 10 mm. at thedrive socket.
 5. The bioabsorbable interference screw of claim 1,wherein the screw has a diameter of 11 mm. at the drive socket.
 6. Thebioabsorbable interference screw of claim 1, wherein the screw has adiameter of 12 mm. at the drive socket.